1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile, a copier, etc.
2. Discussion of the Background Art
Conventionally, a color image forming apparatus that includes plural image formation sections for forming color images of component colors including black is known.
As described in the Japanese Patent Application Laid Open No. 2006-201743, there are provided, in an image forming apparatus, a direct transfer position where a black color image formed in a black image formation section is directly transferred and a secondary transfer position where the other color images primary transferred onto an intermediate transfer belt from the image formation sections for the other colors is secondarily transferred. The secondary transfer position is located upstream of the direct transfer position. The intermediate transfer belt is rotatably suspended by plural rollers and driven by a driving roller among the plural rollers. Further, another belt is provided and is rotatably suspended by plural rollers to carry and convey a printing sheet through the direct and secondary transfer positions. Specifically, the other color images transferred onto the printing sheet at the secondary transfer position are superimposed on the black image at the direct transfer position, whereby a full color image is formed. Carrying and conveying the printing sheet with the belt eliminates deviation in a printing sheet conveyance path extending between the direct and secondary transfer positions, whereby the printing sheet can be stably conveyed therebweteen.
However, intracyclical fluctuation in rotational velocity of the rollers which rotatably suspend the belt does occur, due to either eccentricity or a change of load thereon or the like. Accordingly, the rotational velocity of the belt similarly fluctuates in the same cycle. Since a printing sheet is carried and conveyed by the belt, and the conveyance velocity of the printing sheet similarly varies in accordance with the rotational velocity of the belt and the roller when a phase of the velocity fluctuation of the printing sheet differs between the direct and secondary transfer positions, positional deviation occurs between the location of black image and the images formed with the other colors at the direct and secondary transfer positions.
A system excluding the belt can be provided, in which a roller is employed instead of the belt and opposes the drive roller via the intermediate transfer belt to pinch a printing sheet therebetween. Specifically, the roller applies conveyance force and conveys the printing sheet through the secondary transfer and direct transfer positions.
However, the conveyance velocity of the printing sheet fluctuates with a rotation cycle of the roller 13. Thus, when the velocity fluctuates and a phase thereof is different between the direct and secondary transfer positions, displacement of the black from the other colors and vice verse occurs on the printing sheet transferred at these positions in a rotation cycle of the roller.